Abscisic acid is a naturally occurring plant hormone which acts primarily to inhibit growth, maintain dormancy of buds, promote fruit maturation or coloration, activate the pathogen resistance response defense, induce senescence in already-damaged cells and their proximate neighbors, and help the plant tolerate stressful conditions. See Arteca, R. (1996), Plant Growth Substances: Principles and Applications. New York: Chapman & Hall; Mauseth, J. D. (1991), Botany: An Introduction to Plant Biology. Philadelphia: Saunders. pp. 348-415; Raven, P. H., Evert, R. F., and Eichhorn, S. E. (1992), Biology of Plants. New York: Worth. pp. 545-572.
Abscisic acid owes its name to the belief that this plant growth regulator causes the abscission of leaves from deciduous trees in the fall. Absicin II and dormin are names previously used for this plant hormone. The chemistry and physiology of abscisic acid and its analogs is described by Milborrow, Ann. Rev. Plant Physiol. 1974, 25, 259-307.
The naturally occurring form of abscisic acid is S-(+)-abscisic acid. It has been reported that R-(−)-abscisic acid also has some biological activities. See, Zeevart J. A. D. and Creelman, R. A. (1988) Metabolism and Physiology of Abscisic Acid, Annu. Rev. Plant Physiol. Plant Mol. Biol. 39, 439-473. The side chain of naturally occurring abscisic acid is by definition 2-cis, -4-trans.
Abscisic acid was first defined in the early 1960s as a growth inhibitor accumulating in abscissing cotton fruit and leaves of sycamore trees photoperiodically induced to become dormant. See, Finkelstein R R, Rock C D (2002), Abscisic Acid Biosynthesis and Response, The Arabidopsis Book Vol. 45, No. 1 pp. 1-48. Since then, abscisic acid has been shown to regulate many aspects of plant growth and development, including embryo maturation, seed dormancy, germination, cell division and elongation. Although abscisic acid has historically been thought of as a growth inhibitor, young tissues have high abscisic acid levels, and abscisic acid-deficient mutant plants are severely stunted because their ability to reduce transpiration and establish turgor is impaired. Exogenous abscisic acid treatment of mutants restores normal cell expansion and growth.
Abscisic acid is thought to initiate its effects on cells through binding to receptor proteins, although their identities and locations are still largely unknown. Activation of the putative receptor(s) causes a chain of events that results in rapid changes in ion channels and slower changes in the pattern of gene transcription. While many individual components of this chain of events have been identified, a complete picture has not yet been obtained.
Commercial formulations comprising abscisic acid are used in the agricultural industry for various purposes, such as to increase crop yield, to advance fruit maturity and color development, to improve stress tolerance, to slow the growth rate, to adjust the flowering phase and for other uses. Abscisic acid has also been reported to possess insect inhibition qualities. See U.S. Pat. Nos. 4,434,180 and 4,209,530 to Visscher. Contents of these patents are herein incorporated by reference. Abscisic acid in a powdered form is currently commercially available from Lomon Biotechnology Company, Ltd., a Chinese company, which markets it as a substance that, among other uses, improves the yield and quality of certain crops.
However, one of the problems associated with industrial use of abscisic acid formulations is relatively poor storage stability of solvent-based abscisic acid liquid formulations, hydroxylation inactivation of 8′ and 9′ methyl groups in plants (U.S. Pat. No. 6,004,905) and sunlight induced degradation and isomerization of active 2-cis, 4-trans-S-(+) abscisic acid into the inactive 2-trans, 4-trans-S-(+)-abscisic acid isomer. See, Kamuro Y. 1994. Plant and Chemical Regulation 29: 155-165.
On May 30, 2006, the California Department of Pesticide Regulation (DPR) announced an air quality initiative to reduce pesticide-related emissions of volatile organic compounds (VOC). All pesticide formulations sold in California have to meet <30% VOC content as estimated by thermogravimetry analysis (TGA).
U.S. Pat. No. 6,984,609 to Divesetty et al. discloses water soluble granular compositions of at least one plant growth regulator, preferably a gibberellin, a disaccharide and a surfactant.
Therefore, there is an unmet need in the art for stable S-(+)-abscisic acid formulations for commercial applications as well as for the development of low-VOC solid or liquid formulations to meet regulatory requirements.